At present, in the field of the automobile industries, the number of the front engine-front drive (FF) type automobiles produced has rapidly been increased in order to reduce the weight thereof to a level as low as possible and to ensure a larger housing space within the automobile. In addition, the number of the four wheel drive (4WD) vehicles recently produced has likewise rapidly been increased from the viewpoint of their functionality. In these FF-type and 4WD-type automobiles, the power-transmission and the steering thereof are performed through the front wheels and therefore, the power should smoothly be transmitted to the automobiles even when the driver, for instance, fully manipulate the handle thereof. For this reason, these FF-type and 4WD-type automobiles should be equipped with a constant velocity joint as an essential part for transmitting rotational motions at a constant velocity even when the angle between two shafts which cross each other is variously changed.
Moreover, as the constant velocity joint, there have conventionally been used fixed type constant velocity joints such as Zeppa type ones. In such a Zeppa type one, when the joint transmits a turning force while it takes a working or operating angle, it may generate complicated rolling and sliding motions between the component parts thereof interfitted together. Thus, the fixed type constant velocity joint such as a Zeppa type one must withstand severer lubricating conditions from the viewpoints of, for instance, the gradual increase in the output of engines, the ability of automobiles to run at a higher speed and the reduction of the weight of the constant velocity joint, although it has been required for the fixed type constant velocity joints of this type to undergo more complicated motions. As a result, further problems additionally arise, such that the constant velocity joint should further be improved not only in its durability (flaking life), but also the ability of controlling heat generation during using the same.
Similarly, in case of the sliding type constant velocity joint (such as double-offset type constant velocity joint: DOJ) which has such an ability to absorb any change in the effective length of the drive shaft accompanied by the vertical motions of wheels through the sliding motions (10 to 100 mm) of the balls packaged within the joint in the direction of its expansion, it has been required for such a joint to be improved in the durability (flaking life) and the ability of controlling heat generation.
Conventionally, there have been known, for instance, grease compositions for use in constant velocity joints (see, for instance, Patent Document 1 specified below), each comprising a base oil, a specific diurea thickener, molybdenum dialkyl dithiocarbamate, molybdenum disulfide, a specific sulfur-containing extreme-pressure agent and a sulfur and nitrogen-containing extreme-pressure agent. In addition, there have also been known grease products for use in constant velocity joints (see, for instance, Patent Document 2 specified below), each comprising a base grease prepared by incorporating a thickener to a base oil; and a molybdenum dithiocarbamate soluble in the foregoing base oil as well as a molybdenum dithiocarbamate insoluble in the foregoing base oil.
However, these conventional grease compositions suffer from such a problem that the use thereof is insufficient for effective control of heat generation during using the same, while keeping their excellent durability.                Patent Document 1: JP-A-10-273692 (In particular, Claim 1);        Patent Document 2: JP-A-2003-165988 (In particular, Claim 1).        